One method used in modern, inductor based DC/DC boost converters to control the switching of power through the inductor utilizes a high current, low resistance, precision resistor. The resistor, shown in FIG. 1 as Rsense, is used to measure the current flowing through the inductor as the inductor is charging. The inclusion of this off-chip resistor as part of the DC/DC boost converter can be a significant part of the cost of the boost converter circuitry, and increases system complexity by imposing off-chip requirements on the manufacturer of the appliance rather than the more knowledgeable fabricator of the boost converter itself.
Generally, this precision resistor is not included as part of the integrated circuit that controls the converter. The low impedance of the resistor (in the neighborhood of 50 milliohms) makes its inclusion in such a chip problematic, as chip fabricators have difficulty fabricating a low resistance device with a high degree of required precision. The same requirement for precision also makes the installation of Rsense into the enclosing appliance difficult, as careful traces are required in order to avoid instability in the DC/DC converter. As a result of these factors, the cost of both the resistor itself and assembling the resistor into the final product (the television that contains the DC/DC converter, for example) is high.
In addition, this topology often requires the appliance manufacturer to make use of a low pass filter to screen out the noise produced by the MOSFET (shown in FIG. 1 as S1) as S1 opens and closes. Without such filtering, the noise from the MOSFET can make it difficult to accurately read (and then process) the voltage across Rsense. With the added requirement of such filtering, there is additional cost in both the elements of the low pass filter and the careful assembly of these elements onto the circuit board of the appliance.
In addition, the use of the off-chip resistor requires the chip fabricator to execute modifications to compensate for any remaining noise within the PWM control loop. This becomes more problematic when it is understood that the in-circuit compensation is for an external element of the DC/DC converter circuitry, said element installed by the appliance manufacturer rather than the chip fabricator. These additional frequency components add design complexity and design cost to the boost converter circuitry.
While it is technically possible to put S1 on the same substrate as the rest of the components, this is generally impractical because of its high voltage requirement and low resistance rating. The inclusion of this resultantly large switch within the same substrate as the remaining boost converter components would result in a higher cost per unit that can outweigh all of the other costs outlined previously. It is possible to package S1 and the remaining components—manufactured on different substrates—in the same package, but this too is of negligible added benefit and would impose otherwise absent restrictions on the appliance manufacturer.